<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<style>
   div{
       width: 200px;
       height: 100px;
       border: 2px red solid;
   }
   .a{
       transition: width 2s linear 1s; /* 1秒过后，div在2秒内匀速缓慢的变宽 */
   }
   div:hover{
       width: 400px; /* 触发div变宽 */
   }
</style>

<body>
    <div class="a">hello,拉钩</div>
</body>
</html>